In the formation of a wafer-level chip scale packages (WLCSP), integrated circuit devices such as transistors are first formed at the surface of a semiconductor substrate in a wafer. An interconnect structure is then formed over the integrated circuit devices. A metal pad is formed over, and is electrically coupled to, the interconnect structure. A passivation layer and a first polyimide layer are formed on the metal pad, with the metal pad exposed through the openings in the passivation layer and the first polyimide layer.
Post-passivation interconnect (PPI) is then formed, followed by the formation of a second polyimide layer over the PPI. An under-bump metallurgy (UBM) is formed extending into an opening in the second polyimide layer, wherein the UBM is electrically connected to the PPI. A solder ball is then placed over the UBM and reflowed.
The WLCSP may be bonded to printed circuit boards (PCBs) directly. Conventionally, the WLCSP dies that were directly bonded to the PCBs were small dies. Accordingly, the stresses applied on the solder balls that bond the dies to the respective PCBs were relatively small. Recently, increasingly larger dies need to be bonded to PCBs. The stresses occurring to the solder balls thus become increasingly greater, and the methods for reducing the stresses are needed. However, underfill is avoided to be used in the WLCSP to protect the solder balls. The reason is that if the underfill is applied, the bonding between the dies and the PCBs is not re-workable, and the dies can no longer be removed from the respective PCBs if the dies are defective. As a result, if the underfill is applied, the defective dies cannot be replaced with good dies once bonded, and the entire packages fail.